Current cut-off device for high-voltage direct current with adaptive oscillatory circuit, and control method

ABSTRACT

A current cut-off device for high-voltage direct current, includes: at least one primary mechanical switch placed in a main line between a primary point and a secondary point; a primary surge arrester arranged in parallel with the primary switch; and an oscillatory circuit arranged electrically in parallel with the primary switch and electrically in parallel with the primary surge arrester. The oscillation circuit includes, electrically in series, at least an inductance, a capacitance and an oscillation trigger. The device has, in the oscillation circuit, a controllable device for varying the resistance value inserted in series into the oscillation circuit.

TECHNICAL FIELD

The invention relates to the field of high-voltage DC electrical currenttransmission and/or distribution networks, generally referred to as HVDCnetworks. The invention particularly relates to fault current cut-offdevices intended for such networks.

HVDC networks are in particular envisaged as a solution to theinterconnection of disparate or non-synchronous electricity productionsites. HVDC networks are in particular envisaged for the transmissionand the distribution of energy produced by offshore wind farms ratherthan alternating current technologies, due to lower line losses and tothe absence of impact of the parasitic capacitances of the network onlong distances. Such networks typically have voltage levels on the orderof 100 kV and more.

In the present text, a device in which the nominal operating voltage isgreater than 1,500 V in direct current is considered as a high voltage,for a direct current. Such a high voltage is, in a complementary manner,also qualified as a very high voltage when it is greater than 75,000 V(75 kV) in direct current. Of course, the high voltage field includesthe very high voltage field.

The cut-off of the direct current in such networks is a crucial issuedirectly conditioning the feasibility and development of such networks.

There are known cut-off apparatuses of the mechanical circuit breakertype to achieve the cut-off of the alternating current, that is to saythe cut-off of the current is obtained only by the opening of amechanical switch element. Such a mechanical switch element includes twocontact-making conductive parts which are in mechanical and electricalcontact when the switch element is closed and which separatemechanically when the switch element is open. These mechanical circuitbreakers have several drawbacks when they are crossed by high currents.

In the presence of a significant current and/or voltage, the mechanicalseparation can result in the establishment of an electric arc betweenthe two conductive parts, because of the significant energiesaccumulated in the network that the apparatus protects. As long as theelectric arc remains established through the mechanical separation, thecut-off apparatus does not achieve the electrical cut-off since acurrent continues to flow through the apparatus by the presence of thearc. The electrical cut-off, in the sense of the effective interruptionof the flow of the electrical current, is sometimes particularlydifficult to achieve in a direct current and high voltage context, theseconditions tending to maintain the electric arc. Furthermore, thiselectric arc degrades, on the one hand by erosion, the twocontact-making conductive parts, and on the other hand the surroundingenvironment by ionization. In addition, the current takes some time tostop because of this ionization. This requires maintenance operations onthe cut-off apparatus which are burdensome and expensive.

The fault currents in a HVDC network are particularly violent anddestructive. When a fault generating a high current occurs, it isnecessary to quickly cut it off or possibly to limit it while waitingfor the cut-off to be possible. In addition, the cut-off of the HVDCcurrents is more complex to achieve than that of the alternatingcurrents (AC). Indeed, Upon cut-off of an alternating current, advantageis taken of a zero crossing of the current to achieve the cut-off, whichis not the case with a direct current, in particular HVDC.

PRIOR ART

Various solutions have been proposed to facilitate the current cut-offin an HVDC line. For example, documents WO-2015/078525, US-2017/0178844or DE-2136865 can be cited.

Some solutions use many active semiconductor switching components,mainly thyristors and IGBTs. However, these components have a highprice/power ratio. Excessive use of such semiconductor switchesincreases the cost of the solution.

Documents WO-2015/185096 U.S. Pat. Nos. 4,442,469 and 3,758,790 eachdescribe a current cut-off device for high-voltage DC current. Thesedevices comprise a mechanical primary switch and a mechanical secondaryswitch, interposed successively in the main line between the primarypoint and the secondary point but on either side of an intermediatepoint of the main line, the two mechanical switches being eachcontrolled between an open state and a closed state. However, theproblem of current interruption in a mechanical switch in the presenceof an electric arc between the mechanically separated contacts remainsto be solved.

In addition, it is known to provide for an oscillation circuit arrangedelectrically in parallel with the switch. The oscillation circuit isdesigned and able to generate a zero crossing of the current through theswitch to promote the electrical cut-off through the primary switch whenthe latter is mechanically open. In some known embodiments, theoscillation circuit includes at least an inductance, a capacitance andan oscillation trigger, arranged electrically in series into theoscillation circuit in parallel with the switch in which it is desiredto ensure the effective electrical cut-off. Often, it is provided thatthe capacitance is either pre-charged before the triggering of theoscillation circuit. In this case, the circuit may include a circuit forpre-charging the capacitance. However, this oscillation circuit mustthen be configured so that it can interrupt all fault currents able toarise through the concerned switch, including in particular those havingthe maximum intensity that can be anticipated. However, in use, thereare sometimes fault currents that do not reach this maximum value. Inthis case, an oscillation circuit may turn out to be overdimensioned, inthe sense that the counter-current it generates will be very largecompared to the fault current. It follows that in this case, theoscillation circuit will indeed generate one or more zero crossings ofthe current through the concerned switch, but such a zero crossing canthen occur with too high a rate of variation of the intensity dl/dtthrough the switch. In the presence of a too high rate of variation ofthe intensity dl/dt through the switch, it is possible that theelectrical cut-off does not occur, despite the zero crossing.

The device provided by EP-3.091.626 is of this type, including anoscillation circuit arranged electrically in parallel with the switch.This document does not disclose a cut-off device that allows inserting aresistor in series into an oscillation circuit which, as for him, wouldbe in parallel with the primary switch. Indeed, in an electricalcircuit, two components are in series if they are traversed by the sameelectrical current. Therefore, in EP-3.091.626, the resistor 150 cannotbe in series into the oscillation circuit which is in parallel with theswitch. Indeed, it can be assumed that in EP-3.091.626, when the twoswitches 130, 140 are open, the current flowing through the resistor 150is the same as the one flowing through the LC component 120. However, inthis case, neither the resistor 150 nor the LC component 120 are in anoscillating circuit in parallel with the switch 110. And, when either ofthe switches 120 or 130 is closed, the current through the resistor 150cannot be the same as the one flowing through the LC component 120 sincealmost all of the current flowing through the LC component 120 flowsthrough the branch that includes the switch 120 or 130 which is closed,and not in the resistor 150. In D1, the role of the resistor 150 issimply to allow the charging of the capacitance. The resistor plays norole in the oscillation circuit when it injects current into the mainline.

Some prior art documents have proposed adaptive circuits to adapt thecharacteristics of the oscillation circuit based on the fault current.

Document US-2014/299.579, also published under number DE-10.2011.083514,describes an oscillation circuit including at least two parallelbranches each having a capacitor in series with a capacitor branchswitch. Based on the fault current, it is possible to select the numberof capacitors that will discharge their electricity stored in theoscillation circuit. Thus, this adaptive circuit can adapt the amplitudeof the counter-current which is injected into the main line. In eachbranch of the system, the switch must withstand full voltage, whichrequires expensive and bulky components.

Document WO-2015/166600 describes an oscillation circuit including meansfor adapting the inductance value of an oscillation circuit. Oneembodiment includes several dedicated inductive components, for exampleseveral coils, at least some of which are equipped with a bypass switch.The bypass switches must withstand high voltages, thereby requiringexpensive and bulky components.

However, in the devices according to these two last documents, whichaffect the total capacitance or the total inductance which isimplemented in the oscillation circuit, it is generally necessary toprovide for many steps of the total capacitance or total inductancevalue so that the oscillation circuit is capable of interrupting theelectric arc over the whole possible range of the fault currents. Thisleads in practice to multiplying the number of dedicated, inductive orcapacitive components, and even more to multiplying the number ofcontrol switches directly associated with this oscillation circuit. Thisleads to significant cost and space requirement.

In summary, according to the prior art, there is no solution that isboth compact and inexpensive to guarantee effective cut-off of thecurrent in a mechanical switch for a wide range of currents to be cutoff, whether it is a charging current or a fault current, in particularwhen a range of currents to be cut off ranging from a value which may beless than 1,000 amperes up to a value greater than 10 kA is envisaged.

DISCLOSURE OF THE INVENTION

The invention therefore proposes a current cut-off device forhigh-voltage DC electrical current, of the type including:

-   -   a main line between a primary point and a secondary point and        including at least one mechanical primary switch interposed in        the main line;    -   a primary surge protector arranged in parallel with the primary        switch; and    -   an oscillation circuit arranged electrically in parallel with        the primary switch and electrically in parallel with the primary        surge protector, the oscillation circuit including, electrically        in series, at least an inductance, a capacitance and an        oscillation trigger.

According to one aspect of the invention, the device includes, in theoscillation circuit, a controllable device for varying the resistancevalue inserted in series into the oscillation circuit.

A device according to the invention may comprise other optionalcharacteristics of the invention, taken alone or in combination.

The controllable device may be formed of or include at least a bypassswitch and a damping resistor. The bypass switch is able to switchbetween an open state and a closed state, and the damping resistor andthe bypass switch are arranged such that, in a state of the bypassswitch, the damping resistor is inserted electrically in series into theoscillation circuit with the inductance, the capacitance and theoscillation trigger of the oscillation circuit while, in the other stateof the bypass switch, the damping resistor is short-circuited relativeto the oscillation circuit.

The oscillation circuit may include at least one permanent resistor,permanently inserted into the oscillation circuit, electrically inseries with the inductance, the capacitance and the oscillation triggerof the oscillation circuit.

The oscillation circuit may include several damping resistors eachassociated with a distinct bypass switch of the damping resistor, eachbypass switch being able to switch between an open state and a closedstate, and a damping resistor and the associated bypass switch beingarranged such that, in a state of the open-circuit switch, the dampingresistor associated with the switch is inserted electrically in seriesinto the oscillation circuit with the inductance, the capacitance andthe oscillation trigger of the oscillation circuit, while in the otherstate of the bypass switch, the damping resistor associated with theswitch is short-circuited relative to the oscillation circuit.

The cut-off device may include a mechanical secondary switch interposedin the main line such that the primary switch and the secondary switchare interposed successively in the main line between the primary pointand the secondary point but on either side of an intermediate point ofthe main line, the two mechanical switches being each independentlycontrolled between an open state and a closed state.

The cut-off device may include a secondary surge protector arrangedelectrically between the intermediate point and the secondary point,electrically in parallel with the secondary switch.

The cut-off device may include, between the primary point and thesecondary point, a capacitive buffer circuit, electrically in parallelwith the assembly formed by the primary switch and the secondary switch,and electrically in parallel with the assembly formed by the primarysurge protector and the secondary surge protector, the capacitive buffercircuit including an activation switch and a buffer capacitance.Typically, the capacitive buffer circuit does not include a dedicatedinductive component.

The activation switch and the buffer capacitance may be arrangedelectrically in series in a line of the capacitive buffer circuit goingfrom the primary point to the secondary point.

The capacitive buffer circuit may include a circuit for discharging thebuffer capacitance.

The capacitive buffer circuit may include a tertiary surge protectorarranged in parallel with the activation switch, for example directlyacross the activation switch.

The primary switch may include at least one vacuum switch.

The secondary switch may include at least one isolating gas switch ormay include at least one vacuum switch.

The invention also relates to a method for controlling a cut-off device,characterized in that it includes determining a value of intensity of acurrent to be cut off through the device, and determining, based on thedetermined value of fault current intensity, the state into which the atleast one bypass switch must be switched. In such a method, all thebypass switches of the oscillation circuit can be switchedsimultaneously, or on the contrary can be switched with a time shiftrelative to each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a first embodiment of a cut-off deviceaccording to the invention.

FIG. 2 is a schematic view of a second embodiment of a cut-off deviceaccording to the invention

FIG. 3 is a graph schematically illustrating the variations of some ofthe quantities characteristic of the operation of a device according tothe first embodiment of the invention, during an opening process.

FIG. 4 is a schematic view of one variant of an oscillation circuit ableto be implemented in a cut-off device according to the invention.

FIG. 5 is a schematic view of another variant of an oscillation circuitable to be implemented in a cut-off device according to the invention.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is a schematic representation of a first embodiment of a cut-offdevice 10 according to the invention, for high-voltage, even veryhigh-voltage, DC current.

As can be seen in FIG. 1 , the current cut-off device 10 includes aprimary point, which may be a first terminal 12, and a secondary point,which may be a second terminal 14. This primary point 12 and thissecondary point 14, or terminals, form inputs/outputs for the current inthe device 10. Each of these points may correspond to a physicalterminal of the device 10, for example a physical connection terminal,or a virtual terminal of the device 10 as being a point along aconductor.

The device 10 of FIG. 1 includes a main line 16 which extends betweenthe first terminal 12 and the second terminal 14 and in which areinterposed, successively in the main line between the primary point 12and the secondary point 14, a primary switch 18, having a first terminal20 and a second terminal 22, and a secondary switch 24, also having afirst terminal 26 and a second terminal 28. The first terminal 20 of theprimary switch 18 is at the same electric potential as the primary point12. The second terminal 28 of the secondary switch 24 is at the sameelectric potential as the secondary point 14. The second terminal 22 ofthe primary switch 18 and the first terminal 28 of the secondary switch24 are at the same electric potential, and at the same electricpotential as an intermediate point 13 of the main line 16 which isarranged between the two switches 18, 24. When the primary switch 18 andthe secondary switch 24 are in a closed state, letting through theelectrical current, the latter flows through the device 10 in the mainline 16, which is then the line of lowest impedance of the device 10between the primary point 12 and the secondary point 14. Either of theprimary switch 18 and the secondary switch 24, or both, can be switchedinto an open state or a closed state.

The device 10 is intended to be integrated into an electricalinstallation. For example, the first terminal 12 of the device 10 can beconnected to a portion of the installation which may comprise ahigh-voltage source, for example greater than 100 kilovolts. The secondterminal 14 can for example be connected to a current consuming circuit,for example an electrical charge or an electrical network. In this way,it can be considered that, in the example illustrated, the firstterminal 12 is an upstream terminal, or a current input terminal, whilethe second terminal 14 is a downstream terminal, or a current outputterminal, in the direction of flow of the current. Thus, in thisexample, the main line 16 of the device would be intended to be crossedby the nominal current provided by the DC voltage source. However, thedevice 10 according to the invention is reversible, so that a flow ofthe current through the device in the opposite direction could beprovided.

The electrical installation is provided to operate at a nominal DCvoltage, in the high voltage field, therefore at least greater than1,500 volts, preferably in the very high voltage field, thereforegreater than 75,000 volts. The invention will in particular find anadvantageous application for a cut-off device having the ability to cutoff a current of up to 3,000 amperes, preferably of up to 10,000amperes, even up to 20,000 amperes, at a voltage greater than at least100,000 volts (100 kV).

The primary switch 18 and the secondary switch 24 can be in particularof the circuit breaker, disconnector or fuse type, etc. In the morespecific examples described below, the primary switch 18 and thesecondary switch 24 are for example each formed by a circuit breaker.

The primary switch 18 and the secondary switch 24 are preferably bothmechanical electrical cut-off apparatuses, in which the electricalcut-off is obtained by moving, in particular by spacing apart, twoelectrical contacts or pairs of electrical contacts. In mechanicalapparatuses, the displacement of the electrical contacts is generallyachieved by mechanical, pneumatic, hydraulic or electrical maneuveringmembers or actuators, possibly through motion transfer kinematics. Thisdisplacement can be monitored electronically. As indicated above, in thepresence of a significant current and/or voltage, the mechanicalseparation of the electrical contacts can result in the establishment ofan electric arc between the two electrical contacts of the switch, dueto significant energies accumulated in the network that the apparatusprotects. As long as the electric arc remains established through themechanical separation, the switch does not achieve the electricalcut-off since a current continues to flow through the switch by thepresence of the arc. As will be seen below, the invention provides meansfor ensuring the electrical cut-off, in the sense of the effectiveinterruption of the flow of the electrical current.

The primary switch 18 and/or the secondary switch 24 can each consist ofa single mechanical electrical cut-off apparatus, or can each consist ofseveral mechanical electrical cut-off apparatuses arranged electricallyin series and/or in parallel. It may be an apparatus called “metalenclosed” apparatus where the current supply means (also called“busbar”) are enclosed in a sealed chamber filled with an insulatingfluid. The metal enclosed apparatuses can be in particular designed in amore compact way than the apparatuses where the insulation is achievedin the air.

A mechanical electrical cut-off switch may be in the conventional formincluding in particular two electrodes which are held, by insulatingsupports, in fixed positions remote from the peripheral wall of achamber which is at ground potential. These electrodes are electricallyconnected or electrically separated based on the position of a movableconnection member forming part of one of the electrodes, for example asliding tube actuated by a command. The tube is generally carried by anelectrode, to which it is electrically connected, and the separation ofthe tube from the opposite electrode is able to create an electric arcwhich may be extended during the opening motion of the switch duringwhich the tube moves away from the opposite electrode. A mechanicalelectrical cut-off switch conventionally includes two pairs ofelectrical contacts carried by the tube and the two electrodes. Thefirst pair is the pair through which the nominal current passes in thefully closed position of the apparatus. This contact pair can beassisted by a second pair of contacts, called arcing contact or pair ofsecondary contacts. The two contacts of this pair are intended to remainin direct contact during the separation of the first pair so as tominimize the arcing phenomenon on the first one and thus guarantee agood electrical conduction state in the fully closed position.Conversely, the contacts of the secondary pair separate last and see theestablishment of the electric arc.

In some embodiments, the primary switch 18 is a vacuum switch, orincludes at least one vacuum switch, where the active cut-off members,in particular the electrical contacts, are enclosed in a sealed chamberin which the pressure is lower than atmospheric pressure, in particularless than 100 millibars, in particular less than 10 microbars. Such aswitch has the advantage of being able to ensure a complete electricalcut-off even in the case of a current which has a high intensityvariation rate, it is for which the value of the derivative of theintensity compared to time (dI/dt) is high.

In some embodiments of the invention, the secondary switch 24 is aninsulating fluid switch, or includes at least one insulating fluidswitch, in particular insulating gas switch. This type of switches isparticularly adapted to interrupt high-voltage, even very-high voltage,currents. In such an apparatus, the active cut-off members, inparticular the electrical contacts, are enclosed in a sealed chamber inwhich there is an insulating fluid which can be a gas, commonly sulfurhexafluoride (SF6), but liquids or oils can also be used. The insulatingfluid can be a pressurized fluid, for example at a pressure greater thanor equal to 3 bars absolute. This fluid is chosen for its insulatingnature, in particular so as to have a dielectric strength greater thanthat of dry air at equivalent pressure.

Thus, in some embodiments, including the embodiment which will bedescribed in more detail below, the primary switch 18 is or includes avacuum switch and the secondary switch 24 is or includes an insulatingfluid switch, in particular an insulating gas switch. However, othercombinations are possible, for example a combination in which the deviceincludes a primary switch and a secondary switch of the same technology,in particular both of the vacuum switch type.

As can be seen in FIG. 1 , the device 10 includes a primary surgeprotector 30 arranged in parallel with the primary switch 18 between theprimary point 12 and the intermediate point 13, therefore electricallyin parallel with the primary switch 18, and a secondary surge protector32 arranged electrically in parallel with the secondary switch 24, thesecondary surge protector 32 being therefore arranged electricallybetween the intermediate point 13 and the secondary point 14.

Such surge protectors allow limiting the amplitude of the difference ofpotential across the switch in parallel with which they are arranged. Asurge protector 30, 32, or “voltage surge arester”, is therefore adevice that limits the voltage peaks thereacross. The surge protector30, 32 generally comprises an electrical component which has a variableresistance based on the electrical voltage thereacross. The variation ofthe resistance value is generally not linear with the electrical voltageacross the surge protector 30, 32. Generally, below a transition voltageacross the surge protector 30, 32, the resistance thereof issignificant, with zero or relatively small decrease in its resistancebased on the voltage increase, and the surge protector lets through onlya leakage current, preferably less than 1 ampere (A), or even less than100 milliamps (mA). On the contrary, above the transition voltage acrossthe surge protector, the resistance of the latter decreases rapidlybased on the voltage increase, which reaches a clip voltage value, orprotection voltage, for which the resistance of the surge protectorbecomes low, even very low. In other words, the surge protector acts asa voltage limiter thereacross over the current interval for which it waschosen. It opposes the protection voltage when passing the highestcurrent for which the surge protector has been dimensioned. Below thetransition voltage, it tends to prevent the passage of the current.Beyond the transition voltage, it authorizes the passage of the currentthrough the surge protector for a small increase of the voltagethereacross. As known, the transition voltage is generally not anaccurate value but rather corresponds to a range of transition voltage.However, in the present text, as a definition, the transition voltage ofa surge protector is the voltage for which the surge protector letsthrough a current of 1 ampere (A). The protection voltage is the voltageacross the surge protector when it is crossed by the largest current forwhich it has been dimensioned. Among the surge protectors, lightningarresters are in particular known, which may in particular comprisevaristors and TVS diodes (Transient Voltage Suppressor diodes, such as“Transil™” diodes or TVS diodes). In particular, within the scope of theinvention, the primary surge protector 30 and/or the secondary surgeprotector 32 may each comprise a metal oxyde varistor (or MOV).

Advantageously, as in the illustrated example, it can be provided thatthe primary surge protector 30 is a surge protector whose transitionvoltage is for example comprised in the range from 10,000 volts (10 kV)to 100,000 volts (100 kV). The secondary surge protector 32 will begenerally a surge protector whose transition voltage is greater thanthat of the primary surge protector 30. More specifically, the surgeprotector has preferably a transition voltage such that the transitionvoltage ratio between the secondary 32 and primary 30 surge protectorsis between 1 and 10.

In the example of FIG. 1 , including two successive primary 18 andsecondary 24 switches, the transition voltage of the primary surgeprotector 30 is strictly lower than the nominal voltage of theelectrical installation into which the cut-off device 10 is inserted. Inparticularly optimized embodiments, the primary surge protector 30 andthe secondary surge protector 32 will be chosen such that the sum of thetransition voltage of the primary surge protector 30 with the transitionvoltage of the secondary surge protector 32 is greater than or equal tothe nominal voltage of the electrical installation.

Thus, within the context of the device as the one of FIG. 1 , bychoosing a primary surge protector 30 whose protection voltage is avoltage less than 200 kV, it is ensured that the voltage across theprimary switch 18 remains lower than or equal to this protectionvoltage, which allows using a switch whose cost and space requirementare much lower than the equivalent high-voltage systems. The electricalcut-off at this primary switch 18 is also facilitated.

The primary surge protector 30 and/or the secondary surge protector 32can each be made in the form of an assembly of several discretecomponents arranged electrically in series and/or in parallel. Eachdiscrete component is, for example, a lightning arrester, in particulara varistor, such as a metal oxyde varistor, or a TVS diode. Preferably,the assembly of several discrete components arranged electrically inseries and/or in parallel has, from the point of view of the remainderof the device, the behavior of a single surge protector having anequivalent transition voltage for the assembly and a protection voltagefor the assembly.

FIG. 2 is a schematic representation of a second embodiment of a cut-offapparatus 10 according to the invention, for high-voltage, even veryhigh-voltage, DC current, but of simplified design. The elements commonto both embodiments are designated by the same reference signs, and thedetails relating thereto which are set out above remain applicable tothis second embodiment, unless otherwise indicated.

In this simplified embodiment, the cut-off device 10 includes only, inthe main line 16 which extends between the first terminal 12 and thesecond terminal 14, the primary switch 18. This embodiment thereforedoes not include a secondary switch as described in the firstembodiment. This does not prevent the primary switch 18 from being madein the form of an assembly of several electrical switches arranged inseries or in parallel, but which will be seen, from the point of view ofthe remainder of the circuit, as a single switch. The first terminal 20of the primary switch 18 is at the same electric potential as theprimary point 12. The second terminal 22 of the primary switch 18 is atthe same electric potential as the secondary point 14. When the primaryswitch 18 is in a closed state, letting through the electrical current,the latter flows through the device 10 in the main line 16, which isthen the line of lowest impedance of the device 10 between the primarypoint 12 and the secondary point 14. The primary switch 18 can beswitched into an open state or a closed state.

The primary switch 18 is preferably a mechanical electrical cut-offapparatus. The primary switch 18 can consist of a single mechanicalelectrical cut-off apparatus, or by several mechanical electricalcut-off apparatuses arranged electrically in series and/or in parallel.It may be an apparatus called “metal enclosed” apparatus. In someembodiments, the primary switch 18 is a vacuum switch or includes atleast one vacuum switch.

As can be seen in FIG. 2 , the device 10 includes a primary surgeprotector 30 arranged in parallel with the primary switch 18 between theprimary point 12 and the intermediate point 14, therefore electricallyin parallel with the primary switch 18. In this embodiment comprisingonly the primary switch 18 between the primary point 12 and thesecondary point 14, it is possible to provide that the primary surgeprotector 30 has a transition voltage strictly greater than or equal tothe nominal voltage of the electrical installation.

According to one aspect of the invention, common to both embodiments,the cut-off device 10 includes an oscillation circuit 40 which isarranged electrically in parallel with the primary switch 18. It isnoted that the oscillation circuit 40 is arranged electrically inparallel with the primary surge protector 30. The oscillation circuit 40is designed and able to generate a zero crossing of the current throughthe primary switch 18. In the first embodiment, the oscillation circuit40 is arranged between the primary point 12 and the intermediate point13, to generate a zero crossing of the current only through the primaryswitch 18 and not through the secondary switch 24. In the secondembodiment of FIG. 2 , the oscillation circuit 40 is arranged betweenthe primary point 12 and the secondary point 14.

Such an oscillation circuit 40 aims to promote the electrical cut-offthrough the primary switch 18 when the latter is mechanically open.Indeed, it was seen that even after opening of such a switch, anelectric arc may have been established between the separate contacts ofthe switch, preventing the achievement of an effective electricalcut-off. The zero crossing of the current through the primary switch,generated by the oscillation circuit 40, allows promoting the electricalcut-off through the primary switch 18.

The oscillation circuit 40 includes at least an inductance 42, acapacitance 44 and an oscillation trigger 46, arranged electrically inseries into the oscillation circuit 40 in parallel with the primaryswitch 18, that is to say between the primary point 12 and theintermediate point 13 for the first embodiment of FIG. 1 and between theprimary point 12 and the secondary point 14 for the second embodiment ofFIG. 2 . For both cases, for the operation of such an oscillationcircuit 40, it is advantageous that the capacitance 44 is pre-chargedbefore the triggering of the oscillation circuit 40. In this case, thecircuit 40 may include, in addition, a circuit for pre-charging thecapacitance 44 (not illustrated in the figures).

According to one aspect of the invention, the device includes, in theoscillation circuit 40, at least one damping resistor 48, electricallyin series with the inductance 42, the capacitance 44 and the oscillationtrigger 46 of the oscillation circuit 40, and the oscillation circuit 40includes a controllable device for varying the resistance value insertedin series into the oscillation circuit.

In the two exemplary embodiments illustrated in FIGS. 1 and 2 , it isthus advantageously possible to provide, in such an oscillation circuit40, for at least one damping resistor 48 and at least one bypass switch50 of the damping resistor 48. The bypass switch 50 is able to switchbetween an open state and a closed state. The damping resistor 48 andthe bypass switch 50 are arranged, for example in parallel with eachother, such that, in a state of the bypass switch 50, the dampingresistor 48 is electrically in series into the oscillation circuit 40with the inductance 42, the capacitance 44 and the oscillation trigger46 while, in the other state of the bypass switch 50, the dampingresistor 48 is short-circuited relative to the oscillation circuit 40.

In the examples of FIGS. 1 and 2 , the damping resistor 48 iselectrically in series with the inductance 42, the capacitance 44 andthe oscillation trigger 46, in an electric line of the oscillationcircuit 40 which extends directly and only in parallel with the primaryswitch 18, that is to say between the primary point 12 and theintermediate point 13 for the first embodiment of FIG. 1 , and betweenthe primary point 12 and the secondary point 14 for the secondembodiment of FIG. 2 . The bypass switch 50 is arranged directly andonly in parallel with the damping resistor 48. Thus, when the bypassswitch 50 is in an open state, the damping resistor 48 is electricallyin series into the oscillation circuit 40 with the inductance 42, thecapacitance 44 and the oscillation trigger 46 while, when the bypassswitch 50 is in a closed state, the damping resistor 48 isshort-circuited relative to the oscillation circuit 40.

It is noted that the damping resistor 48 can be made in the form of anassembly of several discrete components arranged electrically in seriesand/or in parallel. The associated bypass switch 50 is then generallyarranged electrically in parallel with the assembly.

The oscillation trigger 46 is a switch, advantageously a semiconductorswitch, although a mechanical switch can also be envisaged, inparticular in a device including only the primary switch as illustratedin FIG. 2 . It is preferably bidirectional. It can thus be, as in theexample of FIG. 1 , made in the form of an assembly in parallel with twothyristors 46 a, 46 b mounted head-to-tail. Such an assembly isanalogous to a TRIAC. However, other semiconductor components could beused, such as IGBTs or other types of controlled spark gaps. For voltagewithstand or current handling reasons, the oscillation trigger 46 can bemade in the form of an assembly of switches arranged electrically inseries and/or in parallel but which can preferably be controlled so asto behave as a single switch vis-à-vis the remainder of the device.

The bypass switch 50 is advantageously a semiconductor switch, althougha mechanical switch can also be envisaged. It is preferablybidirectional. It can thus be, as in the examples of FIGS. 1 and 2 ,made in the form of a parallel assembly of two thyristors 50 a, 50 bmounted head-to-tail. Such an assembly is analogous to a TRIAC. However,other semiconductor components could be used, such as IGBTs or othertypes of controlled spark gaps. For voltage withstand or currenthandling reasons, the bypass switch 50 can be made in the form of anassembly of switches arranged electrically in series and/or in parallelbut can be preferably controlled so as to behave as a single switchvis-à-vis the remainder of the device.

FIG. 4 illustrates a first variant of an oscillation circuit 40 for acut-off device 10 according to the invention. In this variant, theoscillation circuit 40 includes at least one permanent resistor R′,permanently inserted into the oscillation circuit 40, electrically inseries with the inductance 42, the capacitance 44 and the oscillationtrigger 46. This permanent resistor R′ is not associated with a bypassswitch. In this example, the permanent resistor R′ determines a minimumvalue of resistance of the oscillating circuit, when the dampingresistor 48 is short-circuited relative to the oscillation circuit 40.When the damping resistor 48 is inserted into the oscillation circuit40, that is to say in the example when the associated bypass switch 50is open, the resistance value of the oscillating circuit is determinedby the sum of the electrical resistance values of the permanent resistorR′ and of the damping resistor 48. The permanent resistor R′ can be madein the form of an assembly of several discrete components arrangedelectrically in series and/or in parallel.

FIG. 5 illustrates a second variant of an oscillation circuit 40 for acut-off device 10 according to the invention. In this variant, theoscillation circuit 40 includes at least a second damping resistor 48′and at least a second bypass switch 50′ associated with the seconddamping resistor 48. The second bypass switch 50′ is able to switchbetween an open state and a closed state. The second damping resistor48′ and the second bypass switch 50′ are arranged, here in parallel witheach other, such that, in a state of the second bypass switch 50′, thesecond damping resistor 48′ is electrically in series into theoscillation circuit 40 with the inductance 42, the capacitance 44 andthe oscillation trigger 46, and with the first damping resistor 48 ifthe latter is inserted into the damping circuit 40. In the other stateof the second bypass switch 50′, the second damping resistor 48′ isshort-circuited relative to the oscillation circuit 40. When the twodamping resistors 48, 48′ are inserted into the oscillation circuit 40,that is to say in the example when the two associated bypass switches50, 50′ are open, the resistance value of the oscillating circuit isdetermined by the sum of the electrical resistance values of the twodamping resistors 48, 48′.

Of course, the variant of FIG. 5 can be generalized to more than twodamping resistors, and consequently more than two bypass switches. Thus,the oscillation circuit 40 may include several damping resistors eachassociated with a distinct bypass switch of the damping resistor, eachbypass switch being able to switch between an open state and a closedstate. A damping resistor and the associated bypass switch are thenarranged, for example in parallel with each other, such that, in a stateof the bypass switch, the damping resistor associated with the switch isinserted electrically in series into the oscillation circuit 40 with theinductance 42, the capacitance 44 and the oscillation trigger 46 while,in the other state of the bypass switch, the damping resistor associatedwith the switch is short-circuited relative to the oscillation circuit40.

By having several damping resistors each associated with a bypassswitch, it is possible to provide that the bypass switches arecontrolled simultaneously. On the contrary, it is possible to providethat some at least of the bypass switches of the oscillation circuit areswitched with a time shift relative to each other. Thus, it is possibleto adapt the total resistance value of the oscillating circuit to morethan two resistance value steps.

It is noted that the two variants of FIGS. 4 and 5 can be combined in anoscillation circuit including at the same time at least one permanentresistor and several damping resistors, all being inserted in series orable to be inserted in series with each other into the oscillationcircuit.

The role and the advantage of the presence of such an oscillationcircuit 40 will appear in particular from the description of theoperation of a device provided therewith. Reference will be made forthis to FIG. 3 , which illustrates the variations of some parameters inthe device of FIG. 1 during a cut-off operation implemented using such acut-off device 10. However, before describing the role and the advantageof the presence of such an oscillation circuit 40, complementaryelements for different variants of a cut-off device 10 as a whole aredescribed below. These complementary elements are optional.

In some embodiments, such as the one of FIG. 1 , the cut-off deviceaccording to the invention 10 may include, between the primary point 12and the secondary point 14, a capacitive buffer circuit 34, without adedicated inductive component, electrically in parallel with theassembly formed by the primary switch 18 and the secondary switch 24,and electrically in parallel with the assembly formed by the primarysurge protector 30 and the secondary surge protector 32. This capacitivebuffer circuit 34 includes an activation switch 36 and a buffercapacitance 38. In the example illustrated, this circuit thereforecomprises an electrical line 35, one end of which is electricallyconnected to the main line 16 at a point which is at the same electricpotential as the primary point 12 and as the first terminal 20 of theprimary switch 18, and the other end of which is electrically connectedto the main line 16 at a point which is at the same electric potentialas the secondary point 14 and as the second terminal 28 of the secondaryswitch 24. It is in this line 35 that are interposed, electrically inseries, the activation switch 36 and the buffer capacitance 38. Thebuffer capacitance 38 may for example comprise or be formed of one ormore capacitors having a total electrical capacitance C38.

the capacitive buffer circuit 34 can have, like any circuit, a parasiticinductance, resulting in particular from the very nature of theelectrical components it comprises, and resulting from the geometry ofthe circuit. However, in the exemplary embodiment, this capacitivebuffer circuit 34 does not include any dedicated inductive component,that is to say any discrete component having a desired inductivefunction, therefore any component having an inductance greater than aparasitic inductance, in particular any coil or any inductiveferromagnetic core. The capacitive buffer circuit may thus have a verylow inductance, for example less than 50 microhenrys or less than 1microhenry per section of 10 kilovolts of nominal network voltage.

In some embodiments, such as the one of FIG. 1 , the capacitive buffercircuit 34 may include a circuit for discharging the buffer capacitance38. In the example of FIG. 1 , the discharge circuit is a passivedischarge circuit, not including any active component. In this example,the discharge circuit includes a resistor 39 which is arranged inparallel with the buffer capacitance 38. Preferably, the resistor 39 hasa high electrical resistance value R39 such that the dipole whichconsists of the buffer capacitance 38 and of the resistor 39 arranged inparallel, and which is inserted into the electrical line 35, has asignificant time constant compared to an electrical cut-off time in thesecondary switch 24, for example a time constant greater than 50milliseconds, preferably greater than 100 milliseconds. In this example,the time constant is equal to the product R39×C38. Another type ofdischarge circuit, not illustrated, may include at least one activecomponent, such as a controlled switch. Thus, a discharge circuit couldcomprise a controlled switch which would be arranged directly in serieselectrically with the resistor 39, the assembly of these two componentsbeing in parallel with the buffer capacitance 38. When the controlledswitch would be switched to a closed state letting through the current,a discharge circuit would be formed between the two plates of the buffercapacitor 38.

In the embodiment of FIG. 1 , the optional presence, in the capacitivebuffer circuit 34, of a tertiary surge protector 37 arranged in parallelwith the activation switch 36 is noted. This tertiary surge protector 37can be advantageously, as illustrated, arranged directly and only acrossthe activation switch 36, in the sense that it is on the contraryarranged, in the line 35 of the capacitive buffer circuit 34,electrically in series with the buffer capacitance 38. The tertiarysurge protector 37 can be advantageously dimensioned to limit thevoltage across the activation switch 36. For example, it is possible tochoose a surge protector whose protection voltage is in the range from10,000 volts (10 kV) to 100,000 volts (100 kV). Thus, by choosing atertiary surge protector 37 whose protection voltage is in this voltagerange, it is ensured that the voltage across the activation switch 36remains in this voltage range, which allows using a switch whose costand space requirement are much lower than the equivalent systems athigher voltage.

However, in the event of presence of such a tertiary surge protector 37in the capacitive buffer circuit 40, attention will be given preferablyto choosing a tertiary surge protector whose transition voltage isgreater than the protection voltage of the primary surge protector 30.

FIG. 3 illustrates, for a cut-off device as illustrated in FIG. 1switching from a closed state allowing the passage of the currentthrough the device, to an open state electrically insulating the primarypoint 12 from the secondary point 14, the variation over time of thefollowing parameters:

-   -   the voltage V24 across the secondary switch 24;    -   the intensity I24 of the current through the secondary switch        24;    -   the intensity I46 of the current through the oscillation trigger        46;    -   the intensity ISO of the current through the bypass switch 50;    -   the voltage V18 across the primary switch 18;    -   the intensity I18 of the current through the primary switch 18;    -   the intensity I30 of the current through the primary surge        protector 30;    -   the intensity I32 of the current through the secondary surge        protector 32;    -   the intensity I12 of the current through the device 10; and    -   the voltage V1214 across the device 10.

In a method for controlling a cut-off device 10 according to theinvention, with a view to bringing the device from its closed state toits open state, a step is provided comprising the mechanical opening ofthe primary switch 18, and for the embodiment of FIG. 1 , of thesecondary switch 24. The two switches can be opened mechanicallysimultaneously, or successively in any order. In the example of FIG. 3 ,it is assumed that this opening is effective at an instant t0. The stepof opening the switch or the two switches can be triggered under normalcharge, for example with a nominal current provided through the device10, by a simple desire to open the cut-off device for example with aview to electrically insulating a portion of the electrical installationelectrically connected to the primary point 12 of the device 10 relativeto another portion of the electrical installation electrically connectedto the secondary point 14 of the device 10. The step of opening theswitch or the two switches can be triggered in the presence of anelectrical fault in the electrical installation, for example with afault current through the cut-off device 10. This fault current may begreater than the maximum nominal current provided through the device 10.Such an opening in the presence of a fault may result from the detectionof this fault, in particular from the detection of one or moreparameters of the current through the device 10, for example theintensity of the current through the device 10. It is noted that, asdescribed above, it is possible that the mechanical opening of theswitch or the two switches 18, 24 does not allow, on its own, theelectrical opening in the sense of the interruption of the passage ofthe current through the cut-off device 10, because of the establishmentof an electric arc through the primary switch 18 or each of the twoswitches. For the description of the following method, this hypothesisis precisely assumed.

In this hypothesis, the method provides for cutting off the current inthe open primary switch 18 to cause the occurrence, across the primaryswitch, of a voltage greater than the transition voltage of the primarysurge protector 30 suitable for switching it into a current conductionmode.

According to one aspect of the invention, to cut off the current in theopen primary switch 18, either of the variants of the oscillationcircuits 40 as described above according to what will be described belowcan be used. In the example of FIG. 3 , the implementation of theoscillation circuit 40 begins at an instant t1 corresponding to theclosure of the oscillation trigger 46, and it is assumed that thiselectrical opening of the primary switch 18 is effective at the instantt3.

To do so, it is proposed to implement an oscillation circuit 40 asillustrated in FIGS. 1 and 2 or in FIG. 4 or 5 , and as described above.Indeed, thanks to such a device, it is possible to insert, as desired,in a controllable manner, and possibly temporarily, at least one dampingresistor 48 in the oscillation loop formed by the oscillation circuit 40and the main line portion 16 which includes the primary switch 18. Thus,without changing the oscillation capacitance 44, nor its initialcharging level, and without changing the inductance 42, it becomespossible, at a lower cost, to modify the oscillation current which isinjected into the oscillation loop by the oscillation circuit 40.

In some described embodiments, the presence of the bypass switch 50associated with a damping resistor 48 allows, instantaneously,transforming the oscillation circuit 40 from a series RLC circuit into aseries LC circuit, or vice versa. In other embodiments, the presence ofthe bypass switch 50 associated with a damping resistor 48 allows,instantaneously, transforming the oscillation circuit 40 of a series RLCcircuit into another series RLC circuit with a different totalelectrical resistance value.

Thus, based on the fault current flowing in the main line 16, it ispossible, with a view to ensuring the effective electrical cut-off inthe primary switch 18, to inject into the oscillation loop, anoscillation current resulting either from the discharge of a series RLCcircuit or from the discharge of a series LC circuit, or from thedischarge of another series RLC circuit with a different totalelectrical resistance value.

In this way, it is possible to provide for a method for controlling acut-off device 10 including an oscillation circuit 40 as illustrated inFIGS. 1 and 2 or in FIG. 4 or 5 , including, at a given instant,determining at least one parameter of a current to be cut off throughthe device, for example determining a value of intensity of thiscurrent. This determination can be direct, for example by the presenceof a current intensity sensor in the main line portion 16 which includesthe primary switch 18. This determination can be indirect, for exampleby analysis of other parameters of the cut-off device or of theinstallation. This determination can combine both a direct determinationand an indirect determination. This determination can be made before thebeginning of the process of opening the cut-off device, in particularbefore any mechanical opening of the primary switch 18. Thisdetermination can be made after the beginning of the process of openingthe cut-off device, in particular after the mechanical opening of theprimary switch 18. Of course, it is also possible to take into account,for this determination, parameters determined before and after the startof the process of opening the cut-off device 10.

On the basis of this determination, for example based on the determinedvalue of fault current intensity, the control method can determine thestate into which the bypass switch 50 associated with the dampingresistor 48, or with a damping resistor 48, must be switched, and inparticular whether and when it must be switched, this in order to adaptthe total electrical resistance value of the oscillation circuit 40.

The example illustrated in FIG. 3 , relating more particularly to theoperation of a device as illustrated in FIG. 1 , illustrates the casewhere the oscillation circuit 40 is activated by the closure of theoscillation trigger 46 at an instant t1. At this instant, and in theinstants which immediately follow, it is noted that the current ISOthrough the bypass switch 50 remains at zero, which testifies to theopen state of the bypass switch 50, up to an instant t2. Thus, betweenthe instants t1 and t2, the damping resistor 48 is effectively insertedinto the oscillation circuit 40, which is then a series RLC circuit.Thus, the start of the discharge of the oscillation capacitance 44corresponds to the discharge of a series RLC circuit. From the instantt2, the bypass switch 50 is switched to its closed state, so as toshort-circuit the damping resistor 48. Thus, the rest of the dischargeof the oscillation capacitance 44 corresponds to the discharge of aseries LC circuit.

Of course, it is possible to determine the duration of the time intervald1 t between the instants t1 and t2, time interval during which thedamping resistor 48 is actually inserted into the oscillation circuit40. This duration d1 t can be predetermined, or it can be determinedbased on some parameters of the electrical current in the device, inparticular based on parameters of the fault current through the primaryswitch 18.

Thus, the cut-off device 10 including an oscillation circuit 40according to the invention, can be controlled so that, at closure of thetrigger switch 46, at least one damping resistor 48 is inserted into theoscillation circuit 40, or on the contrary short-circuited relative tothis circuit. When the damping resistor 48 is inserted into theoscillation circuit 40, it allows in particular limiting the rate ofvariation of the intensity d(I18)/dt of the current generated by theoscillation circuit 40 in the primary switch 18 at the beginning of thedischarge of the oscillation capacitance 44. In cases where the dampingresistor 48 is actually inserted into the oscillation circuit at closureof the trigger switch 46, it can be chosen to be short-circuited after acertain time interval, as illustrated in FIG. 3 , including before theelectrical cut-off is effective in the primary switch 18, or on thecontrary chosen to be kept inserted into the oscillation circuit for thewhole duration of activation of the oscillation circuit 40.

For a device as illustrated in FIG. 1 , an optimal relationship wasdetermined between the parameters characteristic of the oscillationcircuit

$\begin{matrix}{{d1t} = {{\ln\left( \frac{V44i^{2}*C44}{{{Idef}^{2}*L42} + {\left( \frac{{dI}18}{dt} \right)\max^{2}*L42^{2}*C44}} \right)}*\frac{L42}{R48}}} & \left\lbrack {{Math}.1} \right\rbrack\end{matrix}$

with

-   -   d1 t: Interval of time during which the damping resistor 48 is        actually inserted into the oscillation circuit 40;    -   (dI18/dt) max: maximum value of dI18/dt at the zero crossing of        the current in the primary switch 18 for which the primary        switch 18 can, on its own, ensure an electrical cut-off;    -   V44 i: Initial voltage across the capacitance 44 of the        oscillation circuit;    -   Idef: Amplitude of the current to be cut off through the device        10;    -   R48: electrical resistance value of the damping resistor 48;    -   C44: capacitance value of the capacitance 44 of the oscillation        circuit 40;    -   L72: Inductance value of the inductance 42 of the oscillation        circuit 40.

In a cut-off device as illustrated in FIG. 2 , including only theprimary switch 18, this cut-off of the current through the primaryswitch 18 allows that a voltage occurs thereacross. This voltage isreflected across the primary surge protector 30, which can then play itsrole of limiting the voltage across the primary switch 18, thus limitingthe risk of reigniting the electric arc. The protection voltage of theprimary surge protector 30 is greater than the nominal voltage of thenetwork as long as there is current passing through the surge protector,that is to say as long as the current I30 is different from zero. Thisgoes hand in hand with the absorption of energy into the network. Oncethis energy is absorbed, it is considered that the cut-off device 10 ofFIG. 2 is open, because only a leakage current can flow through thedevice 10 by passing through the primary surge protector 30. For that,it is noted that the voltage across the cut-off device 10 is then equalto the voltage across the primary surge protector 30. This voltage, insteady state when the cut-off device 10 is open, will be generally equalto the nominal voltage of the installation. It is therefore judicious tochoose the primary surge protector 30 such that its transition voltageis greater than or equal to the nominal voltage of the installation.

In a cut-off device as illustrated in FIG. 1 , this cut-off of thecurrent through the primary switch 18 forces the current through thedevice 10 to charge the capacitor 44, causing a voltage risethereacross, which results in the occurrence of this same voltage acrossthe primary surge protector 30, and therefore of the same voltage acrossthe primary switch 18. In the event of a large fault current, thisvoltage reaches, at an instant t4 in FIG. 3 , the transition voltage ofthe primary surge protector 30, whose resistance then varies to limitthe increase of the voltage, which reaches a step. At this stage, it isconsidered that the surge protector 30 becomes conductive for thecurrent. Thus, it can be considered that, from the instant t4, thecurrent through the device 10 passes through the primary surge protector30 but continues to flow through the secondary switch 24 due to thepresence of an electric arc between the contacts of the latter.

In any case, it is observed that the electrical resistance value of thedamping resistor 48 does not need to be large. Therefore, thecomponent(s) forming the damping resistor(s) can be compact andinexpensive. In addition, thanks to this low resistance value, thevoltage value imposed on the bypass switch 50 associated with theresistor is also relatively low. Therefore, the component(s) forming thebypass switch(es) 50 can be compact and inexpensive.

To cause the cut-off of the electric arc in the secondary switch 24, thecapacitive buffer circuit 34 is activated by closing the activationswitch 36, which corresponds to the instant t5 in FIG. 3 . In otherwords, the activation switch 36 is switched to allow, in the capacitivebuffer circuit 34, the passage of a current suitable for charging thebuffer capacitance 38 and diverting the current in the secondary switch24. In the initial state, the buffer capacitance 38 is discharged, forexample by the presence of the discharge circuit which is here made bythe discharge resistor 39. Therefore, and due to the presence of adifference of potential across the primary surge protector 30, thecurrent through the device 10 switches to the buffer circuit 34 tocharge the buffer capacitance 38. At first, it is considered that, inthe example illustrated, the resistance value R39 of the resistor 39 islarge enough to neglect the discharge current through the resistor.Conversely, the value of the electrical impedance of the capacitivebuffer circuit 34 is much lower than the one taken by the current beforet5. This charging time for the buffer capacitance 38, which can beconsidered to last until the instant t6 in FIG. 3 , is particularlyimportant because, during this time, the current through the device 10is essentially conducted by the buffer circuit 34, in the form of thecurrent I36 through the activation switch 36, which has the consequenceof reducing or even canceling the current that flowed through thesecondary switch 24, recalling that it is in a mechanical cut-off state,with its contacts separated from each other. This decrease, or evencancellation, of the current I24 through the secondary switch 24 willadvantageously cause the extinction of the electric arc in the secondaryswitch 24. It is noted that the time interval from the instant t5,during which the current must be diverted from the secondary switch 24to the buffer circuit 34, does not need to be very long, it sufficesthat this time, during which the capacitive buffer circuit 34 conductsthe current, is greater than the time necessary for the deionization ofthe gas present between the separate contacts of the secondary switch24. Indeed, once the gas is de-ionized, the spacing of the contacts ofthe secondary switch 24 is sufficient to prevent re-ignition of the arc.This duration is on the order of a few microseconds, preferably lessthan 20 microseconds.

This diversion duration d2 t, from the instant t5 to the instant t6, forwhich a drop, or even a cancellation, of the current I24 through thesecondary switch 24 is observed, can be adjusted to the durationrequired by a proper dimensioning of the components of the circuit. Ingeneral, an increase in the total electrical capacitance C38 of thebuffer capacitance 38 will tend to increase this diversion duration.

As a first approximation, it can be considered that the diversionduration d2 t, from the instant t5 to the instant t6, is governed by thefollowing law:d2t=Vt30×C38/Idefwith:

-   -   d2 t the desired diversion duration;    -   Vt30 the transition voltage of the primary surge protector 30;    -   C38 the total electrical capacitance of the buffer capacitance        38;    -   Idef the value of the fault current through the device.

Thus, as an indication, it has been determined that an advantageousvalue of the total electrical capacitance C38 of the buffer capacitance38 could be determined by making sure that this value is equal to orgreater than the desired diversion duration d2 t multiplied by themaximum fault current value Idefmax expected through the device, dividedby the transition voltage Vt30 of the primary surge protector 30,namely:C38=d2t×Idefmax/Vt30

Beyond the instant t6, it is considered that the secondary switch 24 iselectrically open and that a voltage may occur thereacross without therisk of reigniting the electric arc. This voltage is reflected acrossthe secondary surge protector 32, which can then play its role oflimiting the voltage across the secondary switch. The sum of thevoltages across the surge protectors 30 and 32 is the voltage V1214.This sum of voltage can be greater than the nominal voltage of thenetwork as long as there is current passing through the surgeprotectors, that is to say as long as the current I32 is different fromzero. This goes hand in hand with the absorption of energy into thenetwork.

From the instant t7, it is considered that the cut-off device 10 isopen, because only a leakage current can flow through the device 10 bypassing through the primary surge protector 30 and through the secondarysurge protector 32. For that, it is noted that the voltage across thecut-off device 10 is the sum of the voltages across the primary surgeprotector 30 and across the secondary surge protector 32. This voltage,in steady state when the cut-off device 10 is open, will be generallyequal to the nominal voltage of the installation. It is thereforejudicious to choose the primary surge protector 30 and the secondarysurge protector 32 such that the sum of their transition voltage isgreater than or equal to the nominal voltage of the installation.

It will be noted that the cut-off device 10 according to the inventioncan be associated, in the electrical installation, electrically inseries with another cut-off device, for example of the disconnectortype, able to completely and reliably interrupt the current in the line.This other cut-off device can be dimensioned to optimize its insulationproperties, without having to optimize its current interruptingcapability since this function will be primarily ensured by the cut-offdevice according to the invention.

It should furthermore be noted that the cut-off device according to theinvention is a bidirectional device, able to interrupt a current flowingthrough the device regardless of its direction of flow, therefore inboth directions through the device. Therefore, such a cut-off devicecould be implemented in an installation including a mesh network, in aline in which the direct current can flow, depending on theconfiguration of the network at a given time, in either direction.

A device according to the invention therefore allows ensuring a rapidand certain electrical opening, to stop the flow of a high-intensityfault current (in particular more than 10 kA), at a high voltage, inparticular greater than 100 kV. However, once the device is open, it isnecessary to be able to electrically re-close the cut-off device 10 inorder to allow the restoration of the current if it is believed that thecause of the fault has been overcome. In the case of the device of FIG.1 , the device 10 is controlled so as to mechanically close the primaryswitch 18 and the secondary switch 24, preferably successively and inthis order, therefore by mechanically closing the primary switch 18before the secondary switch 24. Indeed, it is noted that, by respectingthis order, the secondary surge protector 32 allows limiting the inrushcurrent when the primary switch 18 is mechanically re-closed.

As soon as the primary switch 18 is closed, it is possible to determineone or more parameters of the current through the cut-off device 10and/or of the phase-to-ground voltage, or in the installation, inparticular to verify that the fault has been eliminated. However, thefault may not have been eliminated. Thus, based on the parametersdetected for the current through the device and/or the phase-to-groundvoltage, an immediate reopening of the device can be caused withoutwaiting for the re-closure of the secondary switch 24, which wouldconstitute a complete re-closure of the device 10.

The invention claimed is:
 1. A current cut-off device for high-voltageDC electrical current, comprising: a main line between a primary pointand a secondary point and including at least one mechanical primaryswitch interposed in the main line; a primary surge protector arrangedin parallel with the primary switch; and an oscillation circuit arrangedelectrically in parallel with the primary switch and electrically inparallel with the primary surge protector, the oscillation circuitincluding, electrically in series, at least an inductor, a capacitanceand an oscillation trigger, wherein the device includes, in theoscillation circuit, a controllable device for varying the resistancevalue inserted in series into the oscillation circuit.
 2. The cut-offdevice according to claim 1, wherein the cut-off device includes atleast a bypass switch and a damping resistor, in that the bypass switchis able to switch between an open state and a closed state, the dampingresistor and the bypass switch being arranged such that, in a state ofthe bypass switch, the damping resistor is inserted electrically inseries into the oscillation circuit with the inductor, the capacitanceand the oscillation trigger of the oscillation circuit while, in theother state of the bypass switch, the damping resistor isshort-circuited relative to the oscillation circuit.
 3. The cut-offdevice according to claim 1, wherein the oscillation circuit includes atleast one permanent resistor, permanently inserted into the oscillationcircuit, electrically in series with the inductor, the capacitance andthe oscillation trigger of the oscillation circuit.
 4. The cut-offdevice according to claim 1, wherein the oscillation circuit includesseveral damping resistors each associated with a distinct bypass switchof the damping resistor, in that each bypass switch is able to switchbetween an open state and a closed state, a damping resistor and theassociated bypass switch being arranged such that, in a state of thebypass switch, the damping resistor associated with the bypass switch isinserted electrically in series into the oscillation circuit with theinductor, the capacitance and the oscillation trigger, while in theother state of the bypass switch, the damping resistor associated withthe bypass switch is short-circuited relative to the oscillationcircuit.
 5. The cut-off device according to claim 1, wherein the cut-offdevice includes a mechanical secondary switch interposed in the mainline such that the primary switch and the secondary switch areinterposed successively in the main line between the primary point andthe secondary point but on either side of an intermediate point of themain line, the two mechanical switches being each independentlycontrolled between an open state and a closed state.
 6. The cut-offdevice according to claim 5, wherein the cut-off device includes asecondary surge protector arranged electrically between the intermediatepoint and the secondary point, electrically in parallel with thesecondary switch.
 7. The cut-off device according to claim 5, whereinthe cut-off device includes, between the primary point and the secondarypoint, a capacitive buffer circuit, electrically in parallel with anassembly formed by the primary switch and the secondary switch, andelectrically in parallel with the assembly formed by the primary surgeprotector and a secondary surge protector, the capacitive buffer circuitincluding an activation switch and a buffer capacitance.
 8. The cut-offdevice according to claim 7, wherein the capacitive buffer circuit doesnot include a dedicated inductive component.
 9. The cut-off deviceaccording to claim 7, wherein the activation switch and the buffercapacitance are arranged electrically in series in a line of thecapacitive buffer circuit going from the primary point to the secondarypoint.
 10. The cut-off device according to claim 7, wherein thecapacitive buffer circuit includes a circuit for discharging the buffercapacitance.
 11. The cut-off device according to claim 7, wherein thecapacitive buffer circuit includes a tertiary surge protector arrangedin parallel with the activation switch.
 12. The cut-off device accordingto claim 11, wherein the tertiary surge protector is arranged directlyacross the activation switch.
 13. The cut-off device according to claim1, wherein the primary switch includes at least one vacuum switch. 14.The cut-off device according to claim 1, wherein the secondary switchincludes at least one isolating gas switch.
 15. The cut-off deviceaccording to claim 1, wherein the secondary switch includes at least onevacuum switch.
 16. A method for controlling a cut-off device accordingto claim 2, wherein the method includes determining a value of intensityof a current to be cut off through the device, and determining, based ona determined value of fault current intensity, the state into which theat least one bypass switch must be switched.
 17. The method forcontrolling a cut-off device according to claim 16, wherein all thebypass switches of the oscillation circuit are switched simultaneously.18. The method for controlling a cut-off device according to claim 16,wherein some at least of the bypass switches of the oscillation circuitare switched with a time shift relative to each other.